Gordana A. Milovanović

Spectrophotometric and electrochemical study of protolytic equilibria of some oximes-acetylcholinesterase reactivators.

Newly synthesized oximes, mono and bis imidazole derivatives, which promise to be more effective acetylcholinesterase reactivators than standard antidotes used, were investigated by spectrophotometric and electrochemical methods. The electrochemical investigations confirmed the existence of overlapping equilibria, obtained by spectrophotometric methods. Dissociation constants of those oximes were also obtained by numerical treatment of overlapping equilibria, using the Lavendberg Marquardt least square method, and when compared with the same for some similar compounds, were found to be very effective acetylcholinesterase reactivators. The distribution of ionic forms of the investigated oximes, as a dependence of pH values, was calculated from the obtained values of dissociation constants. The results indicated that many oxime anions will be available at physiological pH 7.4 and a relative increased ability to reactivate inhibited acetylcholinesterase could be expected.

Kinetic determination of codeine in pharmaceutical preparations

A kinetic method for the determination of codeine, based on its inhibitor action on the catalytic decomposition of hydrogen peroxide by cobalt(II), is presented. It has been found that the effect of codeine is most pronounced in the presence of 5% v/v ethylene glycol. The reaction is followed photometrically. Codeine can be determined in concentrations ranging from 0.80×10−5M to 2.4×10−5M. The method has been applied to the determination of codeine in pharmaceutical preparations.

Determination of organic substances by kinetic-catalytic methods of analysis

Abstract A review on the determination of organic substances by kinetic-catalytic methods of analysis is given. Methods based on homogeneous catalyzed solution reactions as well as those based on heterogeneous catalyzed electrode reactions are discussed.

Kinetic method for the determination of traces of thyroxine by its catalytic effect on the Mn(III) metaphosphate-As(III) reaction.

A new, highly sensitive and simple kinetic method for the determination of thyroxine was proposed. The method was based on the catalytic effect of thyroxine on the oxidation of As(III) by Mn(III) metaphosphate. The kinetics of the reaction was studied in the presence of orthophosphoric acid. The reaction rate was followed spectrophotometrically at 516 nm. It was established that orthophosphoric acid increased the reaction rate and that the extent of the non-catalytic reaction was extremely small. A kinetic equation was postulated and the apparent rate constant was calculated. The dependence of the reaction rate on temperature was investigated and the energy of activation and other kinetic parameters were determined. Thyroxine was determined under the optimal experimental conditions in the range 7.0 x 10(-9) to 3.0 x 10(-8) mol L(-1) with a relative standard deviation up to 6.7% and a detection limit of 2.7 x 10(-9) molL(-1). In the presence of 0.08 mol L(-1) chloride, the detection limit decreased to 6.6 x 10(-10) mol L(-1). The proposed method was applied for the determination of thyroxine in tablets. The accuracy of the method was evaluated by comparison with the HPLC method.

A novel kinetic method for the determination of uranium(VI) based on the decomposition of hydrogen peroxide

Abstract A kinetic method is proposed for the determination of uranium(VI), based on its catalytic action on the decomposition of hydrogen peroxide in alkaline media. The reaction rate was followed by the stopped-flow spectrophotometric technique. On the basis of data obtained by kinetic investigations, the kinetic expression was postulated and the conditional rate constant was calculated. The method developed can be applied to the determination of uranium(VI) by the tangent method in the concentration range from 0.8 to 6.4 μg/ml. The method is selective since it enables the determination of uranium(VI) in the presence of 50–1000 times higher concentrations of NH 4 (I), Ca(II), Al(III), Fe(III), Mo(VI), phosphate, sulfate, and fluoride. The method is applied for the determination of uranium in phosphoric acid and phosphate ores.

A kinetic determination of ultramicro quantities of serotonin, 5-hydroxyindolacetic acid, L-dopa, methyl-dopa and carbidopa

SummaryA kinetic method is presented for the determination of the biologically important substances serotonin, 5-hydroxyindolacetic acid, L-dopa, methyldopa and carbidopa. The method is based on the reaction between hydrogen peroxide, molybdenum(VI) and the substance to be determined, in carbonate buffer solution, the reaction rate being followed photometrically. In order to find optimum experimental conditions for these determinations the kinetics of the reactions were examined. The substances investigated were determined at concentrations ranging from 0.40 to 17.4 μg/ml, with relative standard deviations ranging from 0.6 to 6.1%. The effect of some foreign species usually present in biological material has also been examined.ZusammenfassungEine kinetische Methode zur Bestimmung der biologisch wichtigen Substanzen Serotonin, 5-Hydroxyindolessigsäure, L-Dopa, Methyldopa und Carbidopa wurde vorgeschlagen. Sie beruht auf der Reaktion zwischen Wasserstoffperoxid, Molybdän(VI) und der zu bestimmenden Substanz in carbonat-gepufferter Lösung. Das Reaktionsergebnis wird photometrisch gemessen. Um optimale Reaktionsbedingungen zu ermitteln, wurden die kinetischen Daten geprüft. Die genannten Substanzen wurden in Konzentrationen zwischen 0,40 und 17,4 μg/ml bestimmt, wobei relative Standardabweichungen zwischen 0,6 und 6,1% auftraten. Die Wirkung mancher, in biologischem Material üblicherwiese anwesender Fremdsubstanzen wurde geprüft.

Kinetic-spectrophotometric method for the determination of neodymium and mixtures of neodymium with lanthanum and cerium

A kinetic method is described for the determination of neodymium, based on its inhibitory effect on the acetonitrile-catalysed oxidation of Pyrocatechol Violet by hydrogen peroxide. The reaction rate was followed spectrophotometrically at 490 nm. The method allows the determination of neodymium in the concentration range 0.14–1.4 µg ml–1 with a relative standard deviation of about 4%. The effect of lanthanum and cerium on the reaction inhibited by neodymium, which is manifested by an increase in this effect, was used for their determination in mixtures with neodymium. Binary and ternary mixtures of neodymium, lanthanum and cerium were analysed with a relative standard deviation of up to 6.1%.

Kinetic determination of trace levels of selenium(IV) and total selenium by nile blue A/hydrogen peroxide method

A kinetic method for the determination of selenium(IV) traces is proposed, based on its inhibitory action on the oxidation of Nile Blue A by hydrogen peroxide in phosphate buffer (pH 10.5). A linear dependence was established between the rate of the proposed indicator reaction and selenium concentration in the range 9.5 × 10−2-1.58 ng cm−3. The experimental conditions of maximal selenium effect were established. Selenium, determined by the tangent method, was determined at concentrations over the range 0.22–1.26 ng cm−3, with relative standard deviations up to 4.5%. The reaction rate was followed spectrophotometrically. The effect of foreign ions on the accuracy of this method was also investigated. The method was applied to the determination of selenium in pharmaceutical preparations and wheat flour.

Photoluminescence of veratridine and its fluorometric determination

Abstract The photoluminescence which includes two concurrent processes, i.e., fluorescence and phosphorescence, of veratridine (a compound belonging to the group of ceveratrum alkaloids) has been established. The photoluminescence excitation and emission spectra of veratridine have been measured and the effects of pH, the buffer amount, and the type of organic solvent, respectively, on the intensity of veratridine fluorescence have been investigated. Optimum conditions for the fluorometric determination of veratridine in ethanol (λex = 305 nm, λem = 365 nm) and the concentration quenching the fluoresence (2 × 10−5 M), have been determined. The method has been applied to the assay of veratridine content in veratrine for the concentration range from 1.83 to 10.84 μg/ml, with a maximum relative standard deviation of 2.1%, without prior separation of veratridine from other components in the mixture.

A kinetic determination of ultramicroquantities of some biologically active substances

Abstract A new kinetic method for the determination of ultramicroquantities of adrenaline, noradrenaline, thyroxine, and 5-hydroxytryptophan is presented. The method is based on the effect of these organic substances on the oxidation of pyrocatechol violet with hydrogen peroxide, catalyzed by Cu(II) ions. In order to find out the experimental conditions under which this effect is optimum, the kinetics of the above-mentioned indicator reaction in the presence of the compounds to be determined was studied in detail. Adrenaline and noradrenaline were determined at concentrations ranging from 2.0 × 10−6 to 8.0 × 10−6 M, thyroxine at concentrations varying from 1.0 × 10−6 to 10.0 × 10−6 M, and 5-hydroxytryptophan over the concentration range 7.7 × 10−7–32.0 × 10−7 M. In these determinations the standard deviation was lower than 10%.